Immortal Soul

Mundus was not stupid. After hearing what Roy said, he immediately understood that this unfamiliar demon lord was here to find trouble with him.

He knew very well that as Sparda's seal weakened, many demons would enter here from the Abyss. And because of the misleading information he put out, the Ouroboros Marks obtained by these Abyss demons would ultimately transfer to his soul and continuously strengthen his mark.

This powerful mark was the foundation of his immortal soul, but it caused these new Abyss demons to lose their protection.

Mundus could hide it from the other demons of the Demon World, but Sparda was one of the insiders. Since this unfamiliar demon lord was involved with Sparda, it meant that he knew about this matter and came to find trouble with him to get the mark back.